Cutting tools formed of cemented carbide have cutting edges exposed to severe environments such as high temperatures and high loads during cutting work, and the cutting edges are often worn or chipped. Coatings that cover the surfaces of base materials such as cemented carbide have been developed for the purpose of improving cutting performance of the cutting tools. Among others, coatings made of compounds of titanium (Ti) and aluminum (Al) with both or one of nitrogen (N) and carbon (C) (hereinafter also referred to as AlTiN, AlTiCN, and the like) can have high hardness, and it is known that increasing the Al content improves oxidation resistance. Improvement in performance of cutting tools is expected by covering the cutting tools with such coatings.
Unfortunately, Ikeda et al. (Non-Patent Document 1) indicates that when an “AlTiN” or “AlTiCN” coating with an atomic ratio of Al exceeding 0.7 is produced by physical vapor deposition (PVD), the layer structure of the coating changes to the wurtzite crystal structure, resulting in reduction of hardness. Setoyama et al. (Non-Patent Document 2) produced a super multilayer film of TiN/AlN by PVD in order to increase an Al content in an “AlTiN” or “AlTiCN” coating. However, it is reported that when an “AlTiN” or “AlTiCN” coating with a thickness exceeding 3 nm per AlN layer is produced, the layer structure changes to the wurtzite crystal structure, resulting in reduction of hardness.
Studies then have been made to increase the Al content by chemical vapor deposition (CVD). For example, Japanese Patent Laying-Open No. 2015-193071 (Patent Document 1) discloses a hard coating layer formed by CVD. This hard coating layer includes a composite nitride layer or a composite carbonitride layer represented by (Ti1-xAlx) (CyN1-y), in which the layer includes crystal grains having a cubic structure, and the composition of Ti and Al periodically varies along the normal direction to the surface of the tool base material.